In a mobile communication network, Doppler shift occurs when the velocity vector of a transmitting mobile platform differs from the velocity of a receiving mobile platform. For example, when two platforms are stationary with respect to each other (or with respect to a common reference frame) and are communicating with each other, the frequency of the signal received by the receiving platform from the transmitting platform will be the same as the frequency transmitted by the transmitting platform. In this case no Doppler frequency shift exists since the distance between the two platforms remain constant. When the distance between the two platforms is reducing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be higher than the frequency transmitted by the transmitting platform due to the Doppler affect. When the distance between the two platforms is increasing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be lower than the frequency transmitted by the transmitting platform due to the Doppler affect.
It is noted that distance between two platforms can also increase or decrease with time though one of the two platforms is stationary with respect to a common reference frame. This relative distance variation between the two platforms with respect to time will result in a Doppler frequency shift that needs to be accounted for by the receiving platform regardless whether it is the moving platform or the stationary platform with respect to a common reference frame.
For a nominal frequency of f0, the actual frequency at the receiver is f0+Δf, where Δf is the Doppler shift. To accommodate this variation in the received frequency of an electromagnetic wave signal, previously developed systems call for the receiver electronics to accept a wider frequency bandwidth than the nominal frequency bandwidth of the signal. This increases the amount of noise entering the receiver, thereby reducing the signal-to-noise ratio. In addition, the variation in frequency means that the system must use larger guard bands, i.e., unused bands of frequency between each link's nominal frequency and the frequencies of other links. This arrangement wastes bandwidth. Furthermore, because the incoming frequency is not precisely known, the receiving modem must scan over a range of frequencies before it can lock onto the carrier. This reduces the time available for data to be received, especially in time division multiple access (TDMA) systems where the modem must resynchronize at the beginning of every time slot. The above described Doppler shift applies to acoustic signals having a frequency of possibly less than 1 Hz to hundreds of KHz, as well as to electromagnetic wave signals. The Doppler shift has the negative effect of increasing the time to establish a two-way communication link due to longer modem synchronization times, the drawback of necessitating the extra bandwidth needed for guard bands, and serves to increase noise present with the received signal as a result of the use of the guard bands.